This invention pertains to new and improved hydrophilic surfaces and to a process for making the same. It is primarily directed toward new and improved semipermeable membranes having hydrophilic surfaces which are intended to be utilized as micro- or ultra-filtration membranes and to a process for making such membranes.
The expression "semipermeable membrane" as used in the preceding is intended to designate a barrier permeated by pores which are large enough to be capable of passing at least some common solvents and which are sufficiently small as to be capable of blocking or holding back comparatively small particles of entrained, colloidal or dissolved substances. Semipermeable membranes having relatively small pores are normally classified as micro-, ultra-filtration or sterilizing membranes depending upon the sizes of the pores in them. Such pore sizes reasonably correspond to the sizes of the largest particles which are capable of passing through these membranes.
An understanding of the invention set forth in this specification does not require a consideration of the sizes of the pores within semipermeable membranes used in various different applications. However, it is considered that an understanding of the present invention is facilitated by recognition of the fact that the speed with which a liquid can move through a porous membrane is quite important from a commercial standpoint. In general, the greater the ability of a porous membrane to pass a liquid such as water the smaller the equipment necessary in order to process a given volume of a suspension or solution so as to separate the liquid in it.
Normally, a so called "bubble point" test involving the measurement of the pressure required to force air through a membrane which has been wetted with water is employed in order to determine if a membrane is free from, or at least is apparently free from, defects such as discontinuities which might interfere with such a separator. As a consequence of the nature of this bubble point test, it is quite important to utilize a semipermeable filter membrane which is hydrophilic in character. This is because a membrane which has lost its ability to be wetted by water may at least in part exhibit the characteristics of a membrane having a ruptured surface when tested utilizing this bubble point test even though, in fact, such a membrane may be without any type of surface defect.
The speed at which a liquid can move through or be forced through a semipermeable membrane is of course related to many different factors. It is not considered that an understanding of this invention requires a discussion relative to the effects on flow rate of items such as: membrane pore size, the rigidity of the membrane so that the pore sizes will not change in accordance with the applied pressure on the membrane, the dimensional stability of the membrane material or materials against other than pressure caused changes during conditions of use, the use of asymmetric pores so as to limit the lengths of the pores at their smallest diameters within the membrane and various other related considerations.
An understanding of the invention should, however, note a related consideration. Preferably any membrane should be of such a character that the flow rate through the membrane will rapidly rise to a normal "working" value for the membrane within a comparatively short time after the initial use of the membrane. Similarly, the membrane should be of such a character that a stable flow through the membrane will be reached within a limited, comparatively short period after the initial use of the membrane.
Although many different semipermeable membranes useful as micro-, ultra-filtration or sterilizing membranes have been developed and, to varying extents, used in the past, it is not considered that such prior membranes have been as desirable as reasonably possible for any one of a variety of different reasons. Some of such prior membranes have been of such a character as to be incapable of satisfactorily passing the so called "bubble point" test briefly discussed in the preceding. Many of such prior membranes do not possess desired flow characteristics. Frequently, such prior membranes have been of such a character that the flow through them under the initial conditions of use has required comparatively prolonged time periods to stabilize.